Levothyroxine is a thyroid hormone product commercially available in 11 different dose strengths. Only five strengths are permissible on the hospital formulary. The purpose of this prospective, randomized, controlled trial is to determine whether limiting dose strengths of levothyroxine affects physicians' ability to manage patients effectively. This trial will assign participating endocrinologists from the National Institute of Child Health and Human Development and National Institute of Diabetes and Digestive and Kidney Diseases outpatient clinics to restrictive (25, 50, 100, 125, and 150 pg dosage formulations permitted) or nonrestrictive (25, 50, 75, 88, 100, 112, 125, 150, 175,200, and 300 big dose formulations permitted) levothyroxine-prescribing groups. Success in achieving therapeutic objectives as measured by thyroid function studies and clinic visits, medication distribution accuracy, and inventory cost will be compared statistically between groups. Preliminary results from 241 patients Of 33 endocrinologists suggest that the differences in therapeutic success between restricted and nonrestricted thyroid patients were not clinically significant. The compliance, frequency of thyroid function tests, clinical visits, and medication errors were also similar. The restricted formulary; however, was significantly more often associated with complex levothyroxine dosing regimens. Further, the inventory and prescription costs were slightly lower with use of the nonrestrictive formularly system. Presently, 870 patients are followed in this protocol.